This invention relates in general to offshore drilling and production equipment, and in particular to an apparatus for connecting a riser to a platform.
In subsea oil and gas wells, particularly in deep water, the wellheads will be located at the sea floor. Risers connect the wellheads or manifolds to a platform for drilling and production. A variety of systems are employed. In one, the platform floats and is anchored in place. Each wellhead has a riser that extends from the sea floor to the platform. In some systems, the risers will be supported at a lower deck level or keel on the platform, which may be beneath the surface of the sea. Each riser will be supported within a receptacle at the lower deck level or keel. A load shoulder in the receptacle supports the weight of the riser. A tieback connector is lowered from an upper deck level into the receptacle to provide a continuous conduit to the upper deck level.
In such systems, the platforms may be anchored such that the risers are curved in a catenary form. Currents and wave movements cause cyclic loading of the connection between the riser and the receptacle. This can result in fatigue damage to the connection.
In this invention, the connection apparatus is preloaded to resist fatigue damage. The receptacle at the platform has an upper shoulder and a lower shoulder. A hanger is attached to the riser, the hanger having a supporting shoulder that engages the lower shoulder to resist downward pull of the riser. In one embodiment, an upper member, which may be a tieback connector, lands in the receptacle above and in contact with the riser hanger. The upper member has a latch that engages the upper shoulder to prevent upward movement of the upper member. The upper member also has a tensioner that cooperates with the latch to exert a downward preload force on the supporting shoulder of the hanger.
In the first embodiment, the upper member has an outer member that carries a radially expandable latch and lands on the hanger. It also has an inner member that is carried within the outer member. The inner member has a locking surface that engages a locking surface on the latch. These locking surfaces are tapered and threaded in the preferred embodiment. Rotation of the inner member moves the inner member downward against the outer member, pushing the latch member outward. As the latch member moves into engagement with the upper shoulder, a downward force is exerted by the upper shoulder, which creates a downward acting preload that force against the hanger latch.
Alternately, the hanger may include a device for mitigating fatigue damage without an independent upper member. The preload member may comprise a radially deflectable lip mounted to the hanger below the supporting shoulder. The lip is forced into radial interference with the receptacle. In one embodiment, a hydraulically actuated wedge member is moved axially upward between the lip and the riser hanger to force the lip outward into engagement with the receptacle. In another embodiment, the lip is sized for radial interference as the hanger is pulled into the receptacle.